1. Field of the Invention
This invention relates to a packaging material suitable for photosensitive materials, particularly photographic photosensitive materials.
2. Description of the Prior Art
Various types of packaging materials for photosensitive materials have been put to practical use, and various properties are required according to their uses.
As a packaging material for photosensitive materials, it is necessary to have various properties, such as packaging material slitability, light-shielding, gas barrier, moistureproofness, antistatic property, rigidity, physical strength such as breaking strength, tear strength, impact puncture strength, Gelbo test strength and wear resistance, heat sealing properties such as heat seal strength, side welding ability (cut-seal ability), hot tack properties (hot-seal ability) and seal ability of contrasting material, flatness, slipping character and the like. Conventional packaging materials for photosensitive materials are a single layer film of a high pressure low density polyethylene (LDPE) resin kneaded with carbon black, a composite laminated film composed of a LDPE resin film, paper, aluminum foil, and the like.
As the packaging material, particularly the light-shielding sealing bag, for packaging a roll of a photographic photosensitive material and sheets of a photosensitive material having a weight of heavier than 1 kg, the inventor has already disclosed a laminated film composed of a cross laminated film using uniaxially stretched high density polyethylene (HDPE) resin films having a great physical strength and a LDPE resin film containing at least either a light-shielding material or an antistatic agent (Japanese Utility Model KOKOKU No. 19087/1981). The inventor has also disclosed an inexpensive cross laminated film where heat sealing properties and light-shielding ability are improved (Japanese Utility Model KOKOKU No. 20590/1986).
Furthermore, the inventor has disclosed other packaging materials, composed of a laminated film containing a light-shielding film layer composed of linear low density polyethylene (L-LDPE) resin blended with carbon black, having great physical strength, being excellent in heat sealing properties, and being inexpensive (U.S. Pat. No. 4,701,359, Japanese Patent KOKAI No. 18547/1987).
However, the aforementioned conventional laminated film containing a LDPE resin film is weak in physical strength such as tear strength, in spite of being thick. Moreover, the heat sealing properties are inferior, and it is expensive. As a result, the packaging material was difficult to secure light-shielding, moistureproofness and gas barrier because of puncture, tear or separation of heat sealed portions during packaging or transportation.
The packaging materials having a cross laminated film disclosed in Japanese Utility KOKOKU Nos. 19087/1981 and 20590/1986 have a strong physical strength such as tear strength and tensile strength, and they were put to practical use for packaging heavy materials until to recently. However, they are inferior in heat sealing properties, and the physical strength varies due to the uneven thickness of an adhesive layer, the uneven draw ratio of the uniaxially stretched HDPE resin films, or the like. Occasionally, they were punctured, or the heat sealed portion was separated. Moreover, the cross laminated film where a longitudinally uniaxially stretched film was laminated to a laterally uniaxially stretched film so that their orientation axes were crossed was expensive, because two kinds of film molding machines were necessary.
Since the packaging materials having a light-shielding L-LDPE resin film layer disclosed in U.S. Pat. No. 4,701,359 or Japanese Patent KOKAI No. 18547/1987 are inexpensive and excellent in heat sealing properties and physical strength such as tear strength and impact puncture strength, they are excellent as the packaging material for photosensitive materials. However, in the case of packaging a heavy photosensitive material or a photosensitive material having sharp edges, the light shielding L-LDPE resin films were occasionally elongated and became thin due to their low Young's modulus, though they were not punctured nor torn. In this case, light shielding and moistureproofness cannot be secured sufficiently. Therefore, they were put to practical use as a laminated film, laminated to a high Young's modulus heat resistant film such an aluminum vacuum metallized nylon or polyester film which is expensive.